Known field-device display modules for fill level measuring comprise illumination units that may have to be supplied with electrical current, i.e. that require additional energy sources. These modules include, for example, light emitting diodes, cold-cathode fluorescent lamps (CCFLs) or electroluminescent foils.
However, in the case of field devices it can be important to keep the energy consumption to an absolute minimum because the available energy supply is limited.